1. Field of the Invention
The present invention relates to a method for the production of a grain-oriented electrical steel sheet used as an iron core of a transformer or other electric appliances. More particularly, the present invention relates to a method for the production of a grain-oriented electrical steel sheet in which core loss is reduced by imparting it with a mirror surface and keeping it free of precipitates at the surface region.
2. Description of the Prior Art
Grain-oriented electrical steel sheet material for use in various types of electric equipment, mainly transformers, contains 0.8-4.8% Si and has a crystal texture preferentially aligned in the {110}&lt;001&gt; orientation. The required characteristics of a grain-oriented electrical steel sheet are a high magnetic flux density and a low core loss, which are represented by B.sub.8 and W.sub.17/50, respectively. A material for iron cores showing low electric power loss, i.e., a grain-oriented electrical steel sheet having low core loss, is strongly desired from the point of view of environmental protection and energy conservation.
Core loss can be subdivided into eddy current loss and hysterisis loss. The former decreases in proportion to reductions in the width of the magnetic domains of the steel sheet, and the later can be reduced by eliminating hindrances to the movement of magnetic domain walls. Primary causes of this hindrance are uneven or rough surfaces of the steel sheet and the presence of precipitates near the steel surface.
In industrial production of a grain-oriented electrical steel sheet having low core loss, priority had been given to the development of techniques for the magnetic domain refinement. For example, in the case of materials for use in stacked cores, partial or linear microstrains are applied to the final annealed steel sheet by laser-beam irradiation, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 55-18566. Also, in the case of materials for use in wound cores, stress relief annealing is applied to the fabricated core without imparting the effect of the magnetic domain refinement, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-117218. According to the above processes, the overall core loss is reduced due to a large decrease in eddy current loss.
On the other hand, various methods for producing a grain-oriented electrical steel sheet having low hysterisis loss at low cost have been proposed. These are directed to obtaining an even and smooth, or mirror-like, steel surface (hereinafter called a "mirror steel surface"). However, commercial production using these methods has not been realized.
The following describes various conventional proposals for reducing hysterisis loss and explains why these were not commercialized.
An inner oxide layer mainly composed of SiO.sub.2 and a glass film mainly composed of forsterite (Mg.sub.2 SiO.sub.4) are present on the surface of a grain-oriented electrical steel sheet produced by the current production process. The inner oxide layer is formed on the steel surface by decarburization annealing. In addition, the glass film is formed on the above inner oxide layer during the final annealing, which reacts SiO.sub.2 with MgO, to avoid having windings of the coil stick to each other.
Since this glass film is formed based on the above inner oxide layer, the interface between the glass film and the steel sheet is not smooth because of the presence of precipitates. As a result, these precipitates become a hindrance to movement of the magnetic domains. This phenomenon is well known from the various reports, for example, by S. D. Washko, T. H. Shen and W. G. Morris, Journal of Applied Physics., vol. 53, pp 8296-8298. Since then, there have been proposed various methods for dealing with this phenomenon. For example, one of the methods is to prevent glass film formation during the final annealing, and another is to obtain an even and smooth steel surface by chemical or mechanical polishing after removal of the glass film. When coarse and highly pure alumina, which is a non-hydrating oxide, is used as an annealing separator in the final annealing, no glass film is formed on the steel surface of the resulting product. This is disclosed in U.S. Pat. No. 3,785,882.
However, the improvement in core loss shown is only 2% at most because of the residue of precipitates present directly below the steel surface and of the uneven surface after the final annealing.
To achieve a mirror surface by elimination of the remaining precipitates present directly below the steel surface, it is known to use a treatment by chemical or electrolytic polishing, as disclosed in Japanese Unexamined Patent Publication Nos. 49-96920 and 60-39123. These methods are suitable for treating small samples in laboratories, but have not yet been practiced in commercial scale production. This is because of the management of the chemical concentration is very difficult and a waste treatment system is required.
With respect to the production of a grain-oriented electrical steel sheet having a mirror finish steel surface free of precipitates, the present inventors previously proposed a method for preventing the formation of precipitates directly below the steel surface by means of coating an annealing separator mainly composed of alumina after elimination of the oxide layer formed on the decarburized steel sheet by pickling, as described in Japanese Unexamined Patent Publication No. 6-256848. According to this method, core loss can be decreased by 0.1 w/kg at W.sub.17/50, in comparison with the case in which the oxide layer is not eliminated. Although it is possible to practice the pickling at an industrial scale according to the above mentioned method, this requires an additional investment in pickling facilities and increases the production cost. Therefore, a strong need exists for development of a grain-oriented electrical steel sheet, having a mirror surface for decreasing core loss, by a simplified process and at low production cost.